A thermoplastic resin foam, which is light and excellent in heat insulation, flexibility and formability etc., is widely employed in various heat insulators such as a roof insulator or a floor insulator, a cushioning material, and various interior materials such as an interior material for a vehicle.
In general, a method carrying out a step of pre-expanding thermoplastic resins for obtaining expandable beads and then carrying out a step of charging the expandable beads in a mold for secondarily expanding the same and fusing the beads with each other thereby obtaining a molding is known as a method of preparing the aforementioned thermoplastic resin foam.
In the foam molding which is obtained by the aforementioned method of preparing a thermoplastic resin foam, however, strength, particularly bending strength is disadvantageously insufficient. In the bead foam molding which is obtained from the aforementioned expandable beads, for example, a non-expandable thin film is formed on the surface of each bead. Therefore, high compressive strength can be expected in the foam molding which is obtained by this method. In the bead foam molding, however, the aforementioned expandable beads are charged in a mold and thereafter secondarily expanded and fused with each other to be formed, and hence fusion power between the beads is low. When a bending load is applied to the foam molding, therefore, separation and/or breakage are readily caused in the fusion interfaces between the beads, leading to insufficient bending strength.
Japanese Patent Laid-Open No. 4-16330 discloses a method of improving the bending strength of a foam molding which is obtained by the aforementioned bead expanding method. In the method disclosed in this prior art, expandable beads which are obtained by pre-expanding thermoplastic resins are compressively charged in a mold and thereafter degassing steam having a pressure which is higher by at least 0.2 kg/cm.sup.2 than that in the mold is introduced into the mold for secondarily expanding/fusing the same, thereby improving fusion power between the beads. In the method described in this prior art, however, the foam molding obtained by forming is pressurized by the degassing steam. In order to prevent reduction of steam flows between the expandable beads resulting from excess compression, therefore, the pressure of the degassing steam cannot be much increased. Thus, the fusion power between the beads cannot be sufficiently increased, and hence it has been still difficult to obtain a foam molding having sufficient bending strength.
Also in the method described in the aforementioned prior art, in addition, two steps including that of pre-expanding the expandable beads and secondarily expanding and fusing the pre-expanded expandable beads in the mold must be executed, leading to a problem of low productivity.